The invention relates generally to antennas and methods and apparatus for receiving radio signals and for transmitting radio signals in conjunction with an electronic device such as a cellular telephone. In particular, the present invention relates to telescoping antennas for use with such cellular telephones.
Many electronic devices use retractable antennas, that is, antennas which are extendible from and retractable into the housing of the electronic device. In electronic devices such as cellular telephones or other devices, the retractable antenna is electrically connected to a signal processing circuit that is contained within a housing of the cellular telephone on a printed circuit board. In order to optimally operate, the signal processing circuit and the antenna should be interconnected such that the respective impedances are substantially matched, and such that the antenna operates at a predetermined frequency or in a predetermined frequency range. Cellular telephones are becoming physically smaller in size, and this creates a problem with antenna systems used for these types of cellular telephones. The miniaturization causes complex mechanical and electrical connections, and it has been found that retractable antennas that retract into the housing of the cellular telephone are becoming prohibitive from a practical manufacturing standpoint.
U.S. Pat. No. 5,856,808 discloses a single feed point matching system for radiotelephones that includes a retractable antenna and a stationary ferrule contact which are configured to define a coaxial capacitor when the antenna is in an extended position. This prior art antenna still has a drawback because of a shift in frequency between operation in retracted and extended positions. U.S. Pat. No. 5,990,839 discloses a radio transmission apparatus having a retractable antenna for use with a transceiver. The disclosed retractable antenna has a first coil located around a rod antenna in a housing and a second coil connected to an extendible portion of the antenna. The coils disclosed are used for radiation of the signal both in the retracted and extended positions, which reduces the effectiveness of the antenna system.
Consequently, a need exists for a retractable antenna which can be used with cellular devices, which need not be contained within the housing of the cellular device, and which has a minimum frequency shift between a retracted or stowed position, and an extended or deployed antenna position.